Recombination between the X and Y chromosomes and the Sxr region of the mouse

Recombination between the mouse Y chromosome short arm and an additional Y short arm-derived chromosomal segment attached distal to the X chromosome PAR

In a male mouse, meiosis markers of processed DNA double strand breaks (DSBs) such as DMC1 and RAD51 are regularly seen in the non-PAR region of the X chromosome; these disappear late in prophase prior to entry into the first meiotic metaphase. Marker evidence for DSBs occurring in the non-PAR region of the Y chromosome is limited. Nevertheless, historically it has been documented that recombination can occur within the mouse Y short arm (Yp) when an additional Yp segment is attached distal to the X and/or the Y pseudoautosomal region (PAR). A number of recombinants identified among offsprings involved unequal exchanges involving repeated DNA segments; however, equal exchanges will have frequently been missed because of the paucity of markers to differentiate between the two Yp segments. Here, we discuss this historical data and present extensive additional data obtained for two mouse models with Yp additions to the X PAR. PCR genotyping enabled identification of a wider range of potential recombinants; the proportions of Yp exchanges identified among the recombinants were 9.7 and 22.4 %. The frequency of these exchanges suggests that the Yp segment attached to the X PAR is subject to the elevated level of recombinational DSBs that characterizes the PAR.

Mouse model systems to study sex chromosome genes and behavior: relevance to humans

Sex chromosome genes directly influence sex differences in behavior. The discovery of the Sry gene on the Y chromosome (Gubbay et al., 1990; Koopman et al., 1990) substantiated the sex chromosome mechanistic link to sex differences. Moreover, the pronounced connection between X chromosome gene mutations and mental illness produces a strong sex bias in these diseases. Yet, the dominant explanation for sex differences continues to be the gonadal hormones. Here we review progress made on behavioral differences in mouse models that uncouple sex chromosome complement from gonadal sex. We conclude that many social and cognitive behaviors are modified by sex chromosome complement, and discuss the implications for human research. Future directions need to include identification of the genes involved and interactions with these genes and gonadal hormones.

The male-specific histocompatibility antigen, H-Y: a history of transplantation, immune response genes, sex determination and expression cloning

H-Y was originally discovered as a transplantation antigen. In vivo primary skin graft responses to H-Y are controlled by immune response (Ir) genes mapping to the MHC. In vitro T cell responses to H-Y are controlled by MHC class I and II Ir genes, which-respectively, restrict CD8 and CD4 T cells: These can be isolated as T cell clones in vitro. T cell receptor (TCR) transgenic mice have been made from the rearranged TCR genes of several of these, of which that specific for H-Y/Db is the best studied. Non-MHC Ir genes also contribute to the control of in vitro CTL responses to H-Y. The Hya/HYA gene(s) encoding H-Y antigen have been mapped using translocations, mutations, and deletions to Yq in humans and to the short arm of the Y chromosome in mice, where they lie in the deletion defined by the Sxrb mutation between Zfy-1 and Zfy-2. Hya/HYA has been separated from the testis-determining gene, Sry/SRY, in both humans and mice and in humans the azoospermia factor AZF has been separated from HYA. In mice transfection of cosmids and cDNAs mapping to the Sxrb deletion has identified two genes encoding H-Y peptide epitopes. Two such epitopes, H-Y/K(k) and H-Y/D(k), are encoded within different exons of Smcy and a third, H-Y/D(b), by a novel gene, Uty. Peptide elution approaches have isolated a human H-Y epitope, H-Y/HLA-B7, and identified it as a product of SMCY. Each of the Hya genes in mice is ubiquitously expressed but of unknown function. Their X chromosome homologues do not undergo X inactivation.

Deletion of Y chromosome sequences located outside the testis determining region can cause XY female sex reversal

An approach designed to map and generate mutations in the region of the short arm of the mouse Y chromosome, known to be involved in sex determination and spermatogenesis, is described. This relies on homologous Yp-Sxra pairing and asymmetrical exchange which can occur at meiosis in XY males carrying Sxra on their X chromosome. Such exchange potentially generates deficiencies and duplications of Yp or Sxra. Three fertile XY females were found out of about 450 XY offspring from XSxra/Y x XX crosses. In all three, despite evidence for deletion of Y chromosomal material, the Sry locus was intact. Each deletion involved a repeat sequence, Sx1, located at a distance from Sry. Since expression of Sry was affected these results suggest that long range position effects have disrupted Sry action.